I did create some furniture for archicad with blender.
Im not sure though that is should be called gdl or not. Let me explain: It is stored as a gdl file, but a true gdl is a parametric object in my opinion, and what i created was parametrical.
What i did was (i believe) : I exporeted the mesh from blender as a obj file, then in archicad file>open object, set the file type to obj then i selected my exported file, hit open, hit ok and saved the object somewhere to fid later.
I dont know how it is done in modern wersions, i still use 6.0